1. Field of the Invention
The present invention relates to the art of prevention of deterioration of a reversible capacity which occurs when a lithium secondary battery as it is fully charged and has a high voltage is left to stand for a long period of time, and more particularly to an apparatus for preventing a lithium secondary battery from being deteriorated, for use on an electric vehicle which employs such a lithium secondary battery as an electric energy source.
2. Description of the Related Art
It has been confirmed in the art that lithium secondary batteries, unlike other secondary batteries such as nickel-hydrogen secondary batteries, suffer an irreversible deterioration that the fully charged capacity (reversible capacity) of a lithium secondary battery is reduced if the voltage across the lithium secondary battery is higher than a rated voltage (normally 3.6 V at room temperature) when the lithium secondary battery is left to stand while or after it is charged.
The irreversible deterioration is considered to take place because the cathode (of carbon) of the lithium secondary battery is covered with a layer of lithium carbonate (LiCO.sub.3). Though the family of lithium secondary batteries has various types of lithium secondary batteries including lithium cobaltate secondary batteries, manganate lithium cobaltate secondary batteries, etc., and they are deteriorated to different degrees, the irreversible deterioration occurs in all those types of lithium secondary batteries.
Lithium secondary batteries are found as an electric energy source on electric vehicles, for example. In such an application, the instrument panel of electric vehicles has a remaining capacity meter as a counterpart of a fuel meter on gasoline-powered automobiles which are propelled by internal combustion engines.
The remaining capacity meter comprises, for example, a liquid crystal display unit for displaying bars to indicate the remaining capacity of lithium secondary batteries in the range from an empty level to a fully charged level.
Remaining capacity meters on electric vehicles are required to be accurate in the display of the remaining capacity. The inventor of the present application has proposed a process of calculating a quantity of deterioration of a lithium secondary battery charged at a high voltage to correct its remaining capacity, i.e., to reduce the remaining capacity, when the lithium secondary battery is detected as being left to stand at the high voltage, so that the remaining capacity of the lithium secondary battery will be determined with accuracy. For details, reference should be made to Japanese patent application No. 8-302171.
According to the proposed process, it is possible to accurately calculate a reduction in the capacity of the lithium secondary battery due to the quantity of irreversible deterioration of the lithium secondary battery charged at a high voltage, for thereby detecting the remaining capacity of the lithium secondary battery highly accurately.
However, the proposed process fails to prevent the lithium secondary battery from being deteriorated at the high voltage though it can accurately detect the remaining capacity of the lithium secondary battery to correct the fully charged capacity thereof.